The present invention relates generally to power conversion systems. More particularly, the invention relates to a power conversion system used in a rotary power generation system for converting a variable-frequency alternating current produced by a variable-speed rotary power generator into an alternating current with controlled amplitude or frequency for feeding an electric grid.
A rotary power generation system comprises a rotary power generator for generating an alternating current with a variable frequency by rotation of a generator rotor and a power conversion system for converting the variable-frequency alternating current into an alternating current with controlled amplitude or frequency to be supplied to an electric grid. One example of such a rotary power generation system is a variable-speed wind turbine power generation system.
Variable-speed wind turbine power generation systems are one type of renewable energy-based power unit that competes with traditional forms of electric power generation. As a result, variable-speed wind turbine power generation systems depend on cost effective, reliable as well as safe means to capture wind energy and convert it to electrical energy that is suitable for delivery miles away. In operation, variable-speed wind turbine power generation systems include a multiple of rotating blades connected to a rotor shaft that are turned by the wind. The rotation of the blades by the wind spins the rotor shaft to generate a rotational torque or force that drives one or more generators to convert mechanical energy to electrical energy. The rotor shaft and generator are mounted within a housing or nacelle that is positioned on top of a truss or tubular tower. The electrical energy generated in the nacelle is distributed down through the tower to a utility grid via a transformer.
Variable-speed wind turbine power generation systems include generators with rotation speeds that vary with wind speed and which generate an alternating current with a variable frequency. Variable speed wind turbine generators can provide more energy over a range of wind speeds as compared with wind turbine generators requiring a constant speed of operation.
Power conversion systems for variable-speed wind turbine power generation systems typically include a generator-side electronic power converter for converting the variable-frequency alternating current into a direct current at a DC link. A line (or grid) side electronic power converter is also provided for converting the direct current at the DC link into an alternating current with controlled amplitude or frequency for feeding the electric grid. The electronic power converters may be made up of a single full size converter or multiple smaller converters operated in parallel, each of which is referred to as a thread. It is desirable to transmit as much of the wind power to the grid as possible.
During the generation of power in a variable-speed wind turbine power generation system, the generator currents or thread currents can become imbalanced by electrical imbalances in the converter, generator or cables. Such imbalances in current can cause power reduction of the generator to occur which leads to inefficient generation of power by the wind turbine.